1. Field of the Invention
This invention relates to a method of laser emission spectroscopic analysis of steel and an apparatus therefor, and more particularly to improvements in a method of laser emission spectroscopic analysis of steel and an apparatus therefor, wherein a light emitted when a laser beam irradiates the surface of steel is spectrally separated and elements contained in the steel are analyzed quantitatively and which are suitable for use in direct quantitative analysis of impurity elements in the steel such as hot metal, molten steel, slag and the like in a pig iron production process and a steel production process.
2. Description of the Prior Art
With the recent progress of the laser technique in various fields, attempts have been made to utilize lasers as a source of excitation for spectroscopic analysis. More specifically, when a powerful laser beam adapted to focus on the surface of a sample by a focusing lens having a suitable focal length irradiates the surface of the sample, a surface layer is rapidly heated. Particularly, if the laser beam is formed into pulse shapes of several tens of nanoseconds, then such conditions occur that energy is locally poured into the sample before heat is diffused in the sample, whereby melting and evaporation occur. Vapor is further excited by the laser beam to be formed into plasma which emits a light. According to the method of laser emission spectroscopic analysis, this light emitted from the plasma is transmitted to a spectroscope by means of a suitable light introducing system, spectrally separated by a diffraction grating or the like and formed into spectra, and thereafter detected by a photographic film, a photomultiplier tube, a photo-diode or the like, whereby the contents of target elements are determined.
However, heretofore, a ratio between the intensity of a signal and the background (hereinafter referred to as "S/B ratio") has been very low and the variations in the output intensity of lasers have been high, whereby the accuracy of analysis and the exactness have been considerably low. Furthermore, when the method of laser emission spectroscopic analysis is used in practical process analysis or the like on site, there have been such disadvantages that mirrors in a focusing system are contaminated and so on by gases from the surface of the sample and scattered matters after the irradiation by the laser, and the method has not been practiced yet.
Aiming at making this laser emission spectroscopic analysis practicable, the applicant has proposed in Japanese Patent Laid-Open No. 100323/82 a method wherein, excluding white noises immediately after the irradiation by the laser, subsequent line spectra are integrated to improve the accuracy of analysis. However, the intensity of white noise is higher than the line spectrum intensity, and consequently, if a satisfactory intensity would not be given to the line spectra, then effective improvements in accuracy would not be attained.
Furthermore, the applicant has proposed in Japanese Patent Laid-Open No. 76744/83 a method wherein an infrared pulse laser beam irradiates a sample such that an energy density on the surface of the sample becomes 2.0.times.10.sup.9 W/mm.sup.2 or more. However, unless a light emitted by the laser is efficiently and sufficiently taken in, the significance has been low.
Further, the applicant has proposed in Japanese Patent Laid-Open Nos. 114746/81 and 219440/83 that mode control of a laser output and signal processing are effected so as to improve the accuracy. Likewise, unless the light emitted by the laser beam is sufficiently and efficiently taken in, it has been impossible to expect a satisfactory effect.